First, nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, has a high energy density and may thus let a large current flow and generate heat in a case where a breakage in the battery or in the device using that battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous electrolyte secondary battery should prevent more than a certain level of heat generation to ensure a high level of safety.
Safety of a nonaqueous electrolyte secondary battery is typically ensured by imparting to the nonaqueous electrolyte secondary battery a shutdown function, that is, a function of, in a case where there has been abnormal heat generation, blocking passage of ions between the cathode and the anode with use of a separator to prevent further heat generation. More specifically, a nonaqueous electrolyte secondary battery typically includes, between the cathode and the anode, a separator that has a function of, in a case where, for example, an internal short circuit between the cathode and the anode has caused an abnormal current to flow through the battery, block that current and prevent the flow of an excessively large current (shutdown) for prevention of further heat generation. The shutdown is performed such that in a case where a nonaqueous electrolyte secondary battery has been heated to a temperature over the normal operating temperature, the heat melts the separator, thereby clogging the pores present in the separator. The separator preferably (i) remains unbroken by heat even in a case where the temperature inside the battery has been raised to a high temperature after the shutdown and (ii) maintains the shutdown state.
The separator is typically a porous film that contains a polyolefin as a main component and that melts at, for example, approximately 80° C. to 180° C. in a case of abnormal heat generation. A porous film containing a polyolefin as a main component is, however, unable to maintain a film structure at high temperatures not lower than the melting point of the polyolefin, and thus breaks. This lets the cathode and the anode of the battery be in direct contact with each other, possibly leading to a short circuit. Further, a porous film containing a polyolefin as a main component adheres poorly to an electrode. This may eventually decrease the battery capacity and/or degrade the cycle characteristic.
There has been a separator that, in order to prevent a short circuit mentioned above, includes (i) a porous film containing a polyolefin as a main component and (ii) on at least one surface of the porous film, a heat-resistant layer including various resins and fillers.
There has also been a separator that, in order to improve the adhesiveness of the separator to an electrode, includes (i) a porous film containing a polyolefin as a main component and (ii) on at least one surface of the porous film, a porous layer (adhesive layer) containing a polyvinylidene fluoride-based resin.
There has been proposed a separator including (i) a porous film containing a polyolefin as a main component and (ii) a heat-resistant layer formed excellently as a result of adjusting the wettability (critical surface tension) of the porous film and the wettability (critical surface tension) of the heat-resistant layer (Patent Literature 1).
There has also been proposed a separator that, in order to improve the adhesiveness between a heat-resistant layer and a porous film containing a polyolefin as a main component and to improve the adhesiveness between fine particles included in the heat-resistant layer, contains an organic binder (for example, a polyvinylidene fluoride-based resin) in the heat-resistant layer (Patent Literature 1).
In addition, it has been publicly known that, for example, the surface wettability of a separator, that is, the liquid injection easiness for an electrolyte solution during battery assembly, is improved by performing a corona treatment on a surface of the separator, that is, a porous layer mentioned above containing a resin, to introduce a polar functional group into the surface of the porous layer containing a resin.
Second, nonaqueous electrolyte secondary batteries, typified by a lithium ion secondary battery, have a high energy density, and are thus currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
To improve characteristics such as safety of a nonaqueous electrolyte secondary battery, there have been tried various modifications to the separator disposed between the cathode and the anode. A porous film containing a polyolefin, in particular, excels in electrical insulation and exhibits good ion permeability. Such a porous film is in wide use as a separator for a nonaqueous electrolyte secondary battery. There have been made various proposals about such a separator.
Patent Literature 2 discloses a polyolefin-based resin cross-linked foamed product containing a polyolefin-based resin composition prepared by mixing alkenyl sulfonate metal salt and a foaming agent with a polyolefin-based resin. The polyolefin-based resin is crosslinked with electron beams, and has closed cells.
Patent Literature 3 discloses a laminated microporous film including (i) a first microporous film containing a first resin composition and (ii) a second microporous film containing a second resin composition having a melting point lower than that of the first resin composition. The laminated microporous film has a porosity of 50 to 70%.
If the separator is damaged during, for example, an operation of removing a coil wound core from an electrode group during the battery production, the battery will be unable to maintain electronic insulation between the cathode and the anode, which will cause a battery performance defect, with the result of a decrease in productivity of the battery assembly. To detect such defects in advance, battery production typically involves a current leak inspection before injection of an electrolyte solution.
Patent Literature 4 discloses a separator including (i) a heat-resistant porous film containing a heat-resistant resin for reducing the defect rate during the leak inspection, (ii) a first polyolefin porous film covering the entire surface of the heat-resistant porous film on the cathode side, and (iii) a second polyolefin porous film covering the entire surface of the heat-resistant porous film on the anode side. The heat-resistant resin has a melting point or heat distortion temperature higher than the melting point or heat distortion temperature of a polyolefin contained in the first and second polyolefin porous films.
Third, nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, has a high energy density and may thus let a large current flow and generate heat in a case where a breakage in the battery or in the device using the battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous electrolyte secondary battery should prevent more than a certain level of heat generation to ensure a high level of safety.
Safety of a nonaqueous electrolyte secondary battery is typically ensured by imparting to the nonaqueous electrolyte secondary battery a shutdown function, that is, a function of, in a case where there has been abnormal heat generation, preventing passage of ions between the cathode and the anode with use of a separator to prevent further heat generation. More specifically, a nonaqueous electrolyte secondary battery typically includes, between the cathode and the anode, a separator that has a function of, in a case where, for example, an internal short circuit between the cathode and the anode has caused an abnormal current to flow through the battery, prevent that current and prevent the flow of an excessively large current (shutdown) for prevention of further heat generation. The shutdown is performed such that in a case where a nonaqueous electrolyte secondary battery has been heated to a temperature over the normal operating temperature, the heat melts the separator, thereby clogging the pores present in the separator. The separator preferably (i) remains unbroken by heat even in a case where the temperature inside the battery has been raised to a high temperature after the shutdown and (ii) maintains the shutdown state.
The separator is typically a porous film that contains a polyolefin as a main component and that melts at, for example, approximately 80 to 180° C. in a case of abnormal heat generation. However, a separator, which is such a porous film, has insufficient shape stability at high temperatures. This poses a risk that even in a case where the shutdown function is performed, the occurrence of contraction, breakage or the like of the film may cause the cathode and the anode to be in direct or indirect contact with each other, leading to an internal short circuit. Specifically, a separator, which is such a porous film, may not be able to sufficiently prevent abnormal heat generation which is caused by an internal short circuit. This risk has created a demand for separators that are capable of ensuring a high level of safety.
Patent Literature 5 proposes, as a porous film having excellent heat resistance, a porous film configured by, for example, laminating, on a polyolefin microporous film, a heat-resistant porous layer which is made of aromatic polymer such as aromatic aramid.
Along with enlargement of lithium secondary batteries, curls of separators are becoming increasingly evident. In a case where a curl occurs to a separator, handling during production of a battery becomes poor. This may pose problems such as defective winding and assembling failure during the production of the battery. Patent Literature 6, for example, proposes, as a technique for solving the problem, a technique in which a multilayer porous film is prevented from thermal shrinkage and has curl-resistant properties even in a high-temperature environment, the multilayer porous film being obtained by use of a porous layer-forming coating liquid which contains a multilayer porous film copolymer composition and inorganic particles, the multilayer porous film copolymer composition containing a copolymer obtained by copolymerization of a monomer composition containing certain monomers, wherein: the monomer composition contains an unsaturated carboxylic acid monomer at a proportion of less than 1.0% by mass; and Tg of the copolymer, which Tg is calculated based on monomers other than a crosslinkable monomer, is −25° C. or lower.
Fourth, nonaqueous electrolyte secondary batteries (hereinafter referred to as “nonaqueous secondary battery”) such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, has a high energy density and may thus let a large current flow and generate heat in a case where a breakage in the battery or in the device using the battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous secondary battery should prevent more than a certain level of heat generation to ensure a high level of safety.
Safety of a nonaqueous secondary battery is typically ensured by imparting to the nonaqueous secondary battery a shutdown function, that is, a function of, in a case where there has been abnormal heat generation, preventing passage of ions between the cathode and the anode with use of a separator to prevent further heat generation. More specifically, a nonaqueous secondary battery typically includes, between the cathode and the anode, a separator that has a function of, in a case where, for example, an internal short circuit between the cathode and the anode has caused an abnormal current to flow through the battery, prevent that current and prevent the flow of an excessively large current (shutdown) for prevention of further heat generation. The shutdown is performed such that in a case where a nonaqueous secondary battery has been heated to a temperature over the normal operating temperature, the heat melts the separator, thereby clogging the pores present in the separator. The separator preferably (i) remains unbroken by heat even in a case where the temperature inside the battery has been raised to a high temperature after the shutdown and (ii) maintains the shutdown state.
The separator is typically made of a filmy porous base material whose main component is, for example, a polyolefin which melts at approximately 80 to 180° C. when abnormal heat generation occurs. However, there is a possibility that the porous base material containing a polyolefin as a main component cannot maintain a film structure at a high temperature which is equal to or greater than the melting point of a polyolefin and is broken, resulting in direct contact between the cathode and the anode of a battery and so in short-circuit. Furthermore, there is a possibility that since the porous film containing a polyolefin as a main component has poor adhesion property with respect to electrodes, decrease in battery capacity and decrease in cycle characteristics occur.
In order to prevent occurrence of the short-circuit and to improve adhesion property of the separator with respect to the electrode, there has been developed a separator in which a porous layer (adhesive layer) containing polyvinylidene fluoride-based resin is laminated on at least one surface of a porous base material containing a polyolefin as a main component.
For example, Patent Literature 7 discloses that a porosity of a porous layer containing polyvinylidene fluoride-based resin is set to 30 through 60% in consideration of adhesion to electrodes and ion permeability.